A wide variety of foods and other materials are susceptible to loss in quality during storage under atmospheric levels of oxygen. The damage can arise from chemical oxidation of the product, from microbial growth, and from attack by vermin--much of which may be avoided by reducing the oxygen availability in the environment of the materials. In the field of packaging, relatively low-oxygen atmospheres have traditionally been generated by vacuum packing and inert gas flushing. Such methods are not, however, generally applicable for various reasons. For example:
soft porous foods such as cakes cannot be subjected to strong vacuum; PA1 fast filling speeds generally preclude substantial evacuation of or thorough inert gas flushing of food packages; PA1 filling some gas-flushed containers, such as beer bottles often results in occlusion of air; PA1 evacuation or flushing offers no residual capacity for removal of oxygen, which may have desorbed from the food or entered the package by leakage or permeation.
As a consequence there has been much interest in chemical techniques for generating low-oxygen atmospheres and deoxygenating liquid or semi-liquid foods. Thus, there are approaches based on the use of oxidisable solids, for example porous sachets containing iron powder. In another technique, oxidisable MXD-6 Nylon is blended with polyester in the walls of blow-moulded containers--the effectiveness of this depends on the presence of a cobalt salt catalyst, moreover the speed of oxygen removal is limited by the oxygen permeability of the polyester. Further methods include sandwiching crystalline oxidisable material between the layers of multilayer containers, and including a catalyst for the reaction of oxygen with hydrogen in a sandwich arrangement as above or as a deposit on the inner surface of the package.
Heterogeneous systems such as described above do not, however, adequately meet the general needs of the packaging industry, largely because they are often oxygen-sensitive prior to use or can be activated only under restricted conditions of, for example, temperature or humidity. One method which might be used to provide oxygen scavenging in packages as required, is disclosed in Rooney, M. L., Chemistry and Industry, Mar. 20, 1982, pp. 197-198. This method involves the inclusion of a photooxidizable rubber and a photosensitising dye into a polymer film packaging material and then exposing it to visible light. Similar methods are disclosed in Rooney, M. L. and Holland, R. V., Chemistry and Industry, Dec. 1979, pp. 900-901 and Rooney, M. L., Journal of Food Science, Vol. 47, No. 1, pp. 291-2294, 298. However, whilst these methods initiate oxygen scavenging upon illumination, they require constant illumination of the package in order to maintain the scavenging effect. U.S. Pat. No. 5,211,875 proposes an alternative method intended to avoid the problem of oxygen-sensitivity prior to use, involving an oxidizable organic compound (typically 1, 2-polybutadiene)and a transition metal catalyst (typically cobalt salt). Oxygen scavenging is initiated by exposing the composition to an electron beam, or ultraviolet or visible light.